1. Field of the Invention
This invention relates to a light-quantity control device for use in a video camera or the like.
2. Description of the Related Art
Heretofore, a light-quantity control device having a control system which is arranged as shown in FIG. 7 or 8 has been proposed. FIG. 7 shows in a circuit diagram the essential arrangement of the control circuit of the conventional light-quantity control device. Referring to FIG. 7, a motor part 3 is arranged to drive a light-quantity control member. The motor part 3 includes a driving coil 4 for causing a magnet rotor 5 to rotate according to the output of a power amplifier 2 which is arranged to make into a motor driving signal a speed error signal outputted from a differential amplifier 1 which is arranged to compare a light-quantity control signal with a speed control signal; the magnet rotor 5 which is arranged to move the light-quantity control member; a damping coil 6 which is arranged to detect the rotating speed of the magnet rotor 5; and a magnetic sensitive element 7 which is arranged to detect the position of the magnet rotor 5. A signal outputted from the damping coil 6 is inputted to the differential amplifier 1 as the speed control signal via a signal amplifier 9. A linear position detection signal which is outputted from the magnetic sensitive element 7 is converted into a linear aperture-value detection signal and outputted via a differential amplifier 8.
FIG. 8 shows in a circuit diagram the essential arrangement of the control circuit of another light-quantity control device which has been proposed also in the past. In the device, a motor part 13 is arranged to drive a light-quantity control member. The motor part 13 includes a driving coil 14 for causing a magnet rotor 15 to rotate according to the output of a power amplifier 12 which is arranged to make into a motor driving signal a speed error signal outputted from a differential amplifier 11 which compares a light-quantity control signal with a speed control signal; the magnet rotor 15 which is arranged to move the light quantity control member; and a magnetic sensitive element 17 which is arranged to detect the position of the magnet rotor 15. A linear position detection signal which is outputted from the magnetic sensitive element 17 is converted into a linear aperture-value detection signal and is outputted via a differential amplifier 18. Further, the linear aperture-value detection signal is converted into a speed control signal by a differentiation circuit 19. The speed control signal is inputted to the differential amplifier 11.
In each of the conventional devices described, the speed control signal is an output proportional to the rotating speed of the magnet rotor but not proportional to a rate of change per unit time of the area of an aperture (aperture value). Although speed control thus can be performed with a constant strength in relation to the rotating speed of the magnet rotor, the speed control is not uniformly performed in relation to a rate of change per unit time of the quantity of light which is an essential target for the control. In other words, with respect to a change of the quantity of light, an action of the light-quantity control member becomes too slow at a large aperture (on the side of a maximum aperture) and too fast at a small aperture (on the side of a minimum aperture) under the above-stated conventional speed control. Under such a control, hunting tends to take place on the side of a minimum aperture in the event of an excessive quantity of light and, moreover, the response time of the light-quantity control member becomes too slow in shifting the aperture from a maximum position to a minimum position.
Further, in the case of the conventional device, the aperture-value detection signal is a linear position detection signal which is in proportion to the amount of driving of the magnet rotor but not in proportion to a rate of change of the area of an aperture. The accuracy of detecting an aperture value, therefore, excessively degrades on the side of the minimum aperture.
In a case where a diaphragm device of a video camera or the like is in a circular shape having at least three blades, it has been necessary to employ a ring-shaped member 120, as shown in FIG. 12, for opening and closing the diaphragm blades in an interlocking relation. A lever 122 which is connected to the rotation shaft of a motor 121 is arranged to engage the diaphragm blades 125, 126 and 127 through the ring-shaped member 120. Further, there is provided a cover plate 128 which is arranged to prevent the diaphragm blades from coming off the diaphragm unit and also to provide a sliding face for the diaphragm blades.
In accordance with the above-stated conventional arrangement, however, the diaphragm blades 125, 126 and 127 cannot be smoothly turned (opened and closed), because a load for rotating the ring-shaped member 120 is large and because the amount of space in the direction of the plate thickness of the diaphragm blades 125 to 127 cannot be kept constant due to unevenness of the ring-shaped member 120 in the direction of the plate thickness thereof. Besides, a reduction in thickness and size of the device is limited by a space required for the ring-shaped member 120. The conventional arrangement thus has caused difficulty also in reducing the weight of the light-quantity control device.
In addition to these problems, another problem with the conventional arrangement lies in that the presence of the lever 122 in the path of driving force transmission to the diaphragm blades degrades the efficiency of the driving force transmission. A further problem lies in that the presence of the cover plate 128 increases a sliding resistance of the diaphragm blades. A still further problem lies in that these members require spaces for them and thus limits a possible reduction in thickness and size of the device. The presence of these members also makes a reduction in weight difficult.